1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer and the like, utilizing an electrophotographic process for effecting image formation by means of an image forming, method using background area exposure for exposing a non-imaged portion.
2. Related Background Art
In recent years, an image forming apparatus such as a laser beam printer, a copying machine and the like utilizing an electrophotographic process has been noticed for its good image quality and its high speed print-out.
In general, as image forming apparatuses, two types, i.e., a digital type and an analogue type are known. An image forming system of the image forming apparatus of digital type can be generally divided into two methods, i.e., image area exposure method (referred to as xe2x80x9cIAExe2x80x9d hereinafter) for exposing an imaged portion and background area exposure (referred to as xe2x80x9cBAExe2x80x9dhereinafter) for exposing an non-imaged portion (background portion).
In the BAE, since developing is effected with respect to an area of a non-exposure portion where charges remain, normally, developer having charging polarity opposite to that of a photosensitive body is used. Since this relationship is the same as the image forming apparatus of analogue type, the BAE has a merit that a developing mechanism, a cleaning mechanism and developer can be commonly used to the image forming apparatus of analogue type.
On the other hand, in the IAE, since developing is effected with respect to an area where a charged amount is reduced by exposure, developer having the same charging polarity as that of the photosensitive body must be used to effect reversal developing. Although both BAE and IAE systems have been put to practical use, there is limitation due to the photosensitive body and developer used.
In general, microscopical defects are locally created on a surface of the photosensitive body, and, when such a photosensitive body is exposed, the charged amount on a surface of the photosensitive body may be locally reduced at such defect portions. In this case, in the IAE, since the developing is effected with respect to the area where the charged amount is reduced, the developer is also applied to the local defect portions, with the result that minute black spot noise of 0.1 mm or less may be generated in a white background.
To the contrary, in the BAE, since the developing is effected with respect to the area where the charges remain, the developer is not applied to the local defect portions, with the result that, although white void is generated in a black background, the white void is filled with the developer by pressure during a fixing operation, so that there is no problem regarding an outputted image. In this way, regarding the defect of the photosensitive body, the BAE has wider latitude than the IAE.
On the other hand, a transfer/separation ability greatly depends upon transferring efficiency, separating efficiency and latitude of re-transferring. In this case, in the IAE, since potential of the non-imaged portion (background portion) is greater than that in the imaged portion, the BAE has wider latitude than the IAE.
Further, when the cleaning is effected after the transferring, since the potential of the photosensitive body is attenuated, in the IAE system for effecting the developing with respect to the region having low potential, much developer is apt to be adhered to the photosensitive body at a cleaning station. Thus, regarding the cleaning, the BAE also has wider latitude than the IAE.
As mentioned above, the BAE has possibility that it can be designed more easily than the IAE and can provide a stable image forming apparatus having wide latitude.
However, in image formation utilizing light beam illumination, the disadvantage that the latitude of the BAE is made narrow for the following reason.
FIG. 11 shows, at its left half, a condition of one-line of the IAE, i.e., a condition that light beam is ON regarding one-line, and, at its right half, a condition of one-line of the BAE, i.e., a condition that light beam is OFF regarding one-line. In this case, the latitude xcex94V1 of the IAE is VD-Vi and the latitude xcex94VH of the BAE is Vb-V2. Incidentally, V1 is a valley of potential ripple in the BAE and V2 is a top (mountain) of the potential ripple.
As can be seen from FIG. 11, in the BAE, if a spot diameter (diameter in 1/e2 of peak light amount) of the light beam is small with respect to a distance between pixels or if light beam power is too small, gap of potential is generated in the light beam illuminating portion, with the result that V2 is increased to make the latitude smaller. Thus, the spot diameter and power of the light beam with respect to the distance between the scanning lines have lower limits. That is to say, the latitude of the BAE becomes narrower than that of the IAE.
Considering co-ordinates (X, Y) having an X axis indicating a main scanning direction and a Y axis indication a sub scanning direction, according to a report regarding calculation of simulation of light energy distribution (xe2x80x9cInvestigation of Exposure System in Laser Beam Printerxe2x80x9d (Vol. 26, No. 4) written by Naoto Kawamura and Michio Itoh and edited by Society of Electrophotography of Japan, 1987), it was found that, when a ratio between one pixel width a and a spot diameter W of the light beam is 1.6, an optimum condition is obtained.
However, in the electrophotographic process, an electrostatic latent image (image formed by static charge) on the photosensitive body is actually developed. Thus, the exposure amount distribution does not directly correspond to the image, and, therefore, even when the optimum exposure amount distribution is obtained, an optimum image is not always obtained. That is to say, only consideration of the relationship between the one pixel width and the spot diameter of the light beam is insufficient to obtain the optimum image.
An object of the present invention is to provide an image forming apparatus which can prevent fog and thinning of line in a device for exposing a non-imaged portion.
Another object of the present invention is to provide an image forming apparatus which can prevent light memory in a device for exposing a non-imaged portion.
A further object of the present invention is to provide an image forming apparatus comprising an image bearing body, charging means for charging the image bearing body, and exposing means for exposing the image bearing body charged by the charging means, wherein the exposing means illuminates a light beam for each of pixels corresponding to a pixel matrix having plural rows and plural columns, and, when it is assumed that a width of one pixel in the pixel matrix is A and a distribution width in a half value of a maximum potential value of potential distribution formed in OFF of light beam illumination of one pixel is Wv, a following relationship is satisfied:
0.6 less than Wv/A less than 1.4.
A still further object of the present invention is to provide an image forming apparatus comprising an image bearing body, charging means for charging or charging the image bearing body, and exposing means for exposing the image bearing body charged by the charging means, and wherein the exposing means illuminates a light beam for each of pixels corresponding to a pixel matrix having plural rows and plural columns, and, when it is assumed that a spot diameter in 1/e2 of a peak light amount of the light beam in light beam illumination of one pixel is We and a distribution width in a half value of a maximum potential value of potential distribution formed in OFF of light beam illumination of one pixel is Wv, a following relationship is satisfied:
0.5 less than Wv/We less than 1.5.